1. Field of the Invention
The present invention is directed to semiconductor optoelectronic devices and more particularly, to quantum well semiconductor optoelectronic devices having improved carrier injection efficiency and wavelength tuning.
2. Description of the Prior Art
Optoelectronic devices, such as lasers, light emitting diodes and light detectors, are useful in a wide range of applications, including communications systems, surgical instruments, and various electronic devices. Semiconductor optoelectronic devices are based upon the excitation and recombination of charge carriers in a semiconductor material. One such semiconductor optoelectronic device is a quantum well device, wherein a thin layer of semiconductor material, the quantum well active region, is sandwiched between layers of another semiconductor that serve as the sources of charge carriers that are injected into the quantum well. The cladding injection layers have a wider bandgap than the active layer and the quantum well layer is made very thin in order to form discrete energy levels in the active layer by the size quantization effect. In order to emit or absorb light, a quantum transition occurs between the conduction band and the valence band of the active region. Electrons are injected into the active region from one cladding layer and holes are injected into the active region from the other cladding layer. The electrons and holes recombine in the active region resulting in the emission of electromagnetic radiation.
There has been significant interest in developing techniques for improving the efficiency of such devices. The interest has focused on increasing the efficiency of carrier injection into the quantum well and recombination within the quantum well and also to providing wavelength tuning. In addition, it is also desirable to provide efficient device operation at wavelengths in the range of 1.3-10 um, currently of interest for optical communications.
One recent development in quantum well light-emitting technology being explored is resonant tunnelling in superlattice regions. In resonant tunnelling, minibands of the superlattice active region are aligned with energy levels in the cladding injector layers so that carriers tunnel through barrier layers from the injector layer to the collector layer at a single energy level. Helm et al., Physical Review Letters, Vol. 63(1) 1989, disclose sequential resonant tunnelling to provide intersubband emission from semiconductor superlattices. Yuh et al. in Appl. Phys. Lett. 51(18) 1987, disclose a band aligned superlattice laser in which an active region is formed with two minisubbands, the upper minisubband being aligned with the minisubband of the emitter region and the lower minisubband being aligned with the minisubband of the collector region While carrier injection is apparently improved, there is extreme dependence on the formation of the superlattice layers which must be very precise in order to provide the alignment of the manmade minibands.